The present invention relates generally to images incorporating information both at the global level and at the microstructure level and to a method of generating such images. The information at the microstructure level offers, in particular, protection against counterfeiting and may be used as a security feature in documents. The invention also relates to documents comprising security features, and to a method of generating such documents, which may include for example, value bearing commercial instruments, certificates, coupons and personal identification instruments.
The term ‘images’ used herein shall be understood in the broad sense to mean any visual representation of matter that may be printed or displayed on a display, for example text, pictures, photographs, drawings and so on.
A microstructure may comprise microstructure elements such as a text, a logo, an ornament, a symbol or any other microstructure shape. When seen from a certain distance, mainly the global image is visible. When seen from nearby, mainly the microstructure is visible. At intermediate distances, both the microstructure and the global image are visible.
Several attempts have already been made in the prior art to generate images incorporating information at the microstructure level, where from far away mainly the global image is visible and from nearby mainly the microstructure is visible. A prior art method hereinafter called “Artistic Screening” was disclosed in U.S. Pat. No. 6,198,545 and in the article by V. Ostromoukhov, R. D. Hersch, “Artistic Screening”, Siggraph95, Computer Graphics Proceedings, Annual Conference Series, 1995, pp. 219–228. This method requires however significant efforts by graphic designers in order to create the microstructure and is limited to bi-level images, i.e. images in black-white or a single color and white.
A prior art method for incorporating a microstructure into an image by computing color differences is disclosed in European Patent application 99 114 740.6. This method does not modify the thickness of the microstructure according to the local intensity of the image.
Another method hereinafter called “Multicolor Dithering” is disclosed in the article by V. Ostromoukhov, R. D. Hersch, “Multi-Color and Artistic Dithering”, Siggraph'99, Computer Graphics Proceedings, Annual Conference Series, 1999, pp. 425–432. The method allows to synthesize color images incorporating as screen dots a fine microstructure capable of representing various shapes such as characters, logos, and symbols and provides therefore strong anti-counterfeiting features. The publication also presents an iterative technique for equilibrating a dither array, which is however slow and cumbersome and does not always converge to yield a satisfying result. A disadvantage of the aforementioned and other known methods is the significant effort required to synthesize dither matrices incorporating the desired microstructure shapes. These efforts require the skills of a computer scientist for building 3D functions, discretizing them, renumbering the resulting dither values and applying to them an equilibration process.
An additional method for creating microstructures within an image relies on a large dither matrix whose successive threshold levels represent the microstructure and uses standard dithering to render the final image (see Oleg Veryovka and John Buchanan, Halftoning with Image-Based Dither Screens, Graphics Interface Proceedings, years 1988–1999, Ed. Scott MacKenzie and James Stewart, Morgan Kaufmann Publ. In this paper, the authors show how to build a dither matrix from an arbitrary grayscale texture or grayscale image. They mainly apply histogram equilibration to ensure a uniform distribution of dither threshold levels. Texture control is obtained by error-diffusion. However, while their method allows to incorporate text within the microstructure, the typographic character shapes do not vary according to intensity, i.e. the character shapes do not become thin or fat depending on the local intensity. Their method is restricted to black-white or single color target images. The authors do not provide a method to construct a dither matrix starting from a bi-level bitmap incorporating the microstructure shapes.
A further method of embedding a microstructure within an image is described in provisional U.S. patent application No. 60/312,170 (filed Aug. 14, 2001, inventor Huver Hu, which teaches how to transform a grayscale seed image or a bi-level seed image into an array of dot ranking values (similar to a dither matrix) to be used by a PostScript Interpreter for synthesizing the final image incorporating the microstructure. This method is however limited to black-white or to single color output images (bi-level images). In addition, the seed image is preferably a grayscale image (FIG. 10 of patent application No. 60/312,170). With bi-level seed images, the generated microstructure is limited to rather simple shapes (FIG. 13 of patent application No. 60/312,170), since shapes grow at increasing darkness levels from a user specified growth center to the shape given by the bi-level seed image. The shape does not grow beyond 60% darkness: darker levels are produced by the growth of a separate superimposed geometric mask (e.g. a triangle, visible on all dark parts of the wedges in FIGS. 2, 12 and 13 of patent application No. 60/312,170). Furthermore, a manual interactive intervention is required to transform a seed image into an array of dot ranking values.
Another approach for embedding information within a color image relies on the modification of brightness levels at locations specified by a mask representing the information to embed, while preserving the chromaticity of the image (see U.S. Pat. No. 5,530,759). However, since the embedded information is not really used to construct the global image, it cannot be considered a microstructure. If the embedded information incorporates large uniform surfaces, the global image may be subject to significant changes and the embedded information may become visible from a large distance. In addition, the mask is fixed, i.e. its shape does not vary as a function of the local intensity or color.
One further related invention disclosed in U.S. Pat. No. 5,995,638 teaches a method for authenticating documents comprising a basic screen made of microstructures and a revealing screen for creating moire intensity profiles of verifiable shapes. U.S. patent 6,819,775 describes a similar method, where however the basic screen and the revealing screen may undergo geometric transformations, yielding screens of varying frequencies.
The incorporation of microstructures in images has applications not only in the field of generation of artistic images, but also in the field of generation of documents that require protection against counterfeiting. It is known to incorporate microstructures as a security feature in certain printed commercial instruments, such as bank notes, using professional printers and printing techniques on special substrates.
A primary consideration in the generation of printed commercial instruments, such as bank notes, vouchers, transportation tickets, entertainment event tickets and other tickets, coupons or receipts bearing or representing a commercial value, is to provide sufficient safeguards against forgery. The required degree of difficulty in producing a forgery will depend above all on the value, the duration of validity and the generality of the commercial instrument. For example, bank notes which are not related to any specific event and remain valid for many years, require security features that are extremely difficult to reproduce. On the other hand, tickets of relatively limited duration, for example transportation tickets, such as train tickets valid on a certain day for a certain destination, or theatre tickets for a specific show, require lower level security features, as long as they ensure that the instrument is difficult to reproduce in the remaining time to the event or requires excessive technical means or human effort in comparison to the value of the commercial instrument.
Verification of the authenticity of many commercial instruments is often based on a visual control. Although it is easy to provide commercial instruments with unique security features, such as encrypted bar codes or other codes, their verification entails the use of electronic processing means that are unpractical or inefficient in many situations.
In commercial instruments relying primarily on a visual control of authenticity, a common security feature is the provision of special substrates that are difficult or too costly to reproduce for a potential forger in relation to the underlying value of the commercial instrument. A disadvantage of the use of special substrates or special printing techniques is that they do not allow the generation of commercial instruments at sites that are not under the issuer's control, whether directly or indirectly.
In view of the wide-spread use of communications networks, such as the internet or local area networks, there is a demand for enabling the generation of visually verifiable printed commercial instruments, such as transportation tickets and entertainment event tickets, at the buyer's site, for example at home with a PC and standard printer.
In international patent application WO 00/67192, a method of generating a commercial instrument with certain visually verifiable security features for printing on a standard printer is described. In the aforementioned application, data relevant to the commercial instrument are manipulated in accordance with predetermined rules to generate a pattern which is visually recognizable to an informed person. The security against forgery of an instrument generated according to the latter method relies on the potential forger's ignorance of the predetermined rules.
Reliance on predetermined rules has a number of disadvantages. Firstly, the rules must be communicated to persons responsible for controlling authenticity, which becomes impractical where many controllers are involved. Secondly, the rules must result in features that are visually recognizable, with the consequence that a potential forger could, on the basis of a number of commercial instruments, be able to deduce the rules with a sufficient degree of approximation to generate forgeries using different data. In this regard, it should be noted that the relatively sophisticated image creation and editing software widely available and for use on PC provide the forger with fairly powerful tools to reproduce images and text manipulated in order to emulate visually recognizable patterns provided on authentic commercial instruments on the basis of predetermined rules as described in international application WO 00/67192.